M400 .223 Rem or 5.56x45mm Nato

This is a discussion on M400 .223 Rem or 5.56x45mm Nato within the SIG Sauer Rifles forums, part of the SIG Sauer Forum category; I'm hearing some are using .223 (which is suppose to be the 5.56 equivalant) and having miss fires, jams, etc but never anyone that uses ...

You can shoot .223 in a rifle chambered for 5.56 but you should not shoot 5.56 out of a rifle chambered in .223. That being said, the M400 is chambered for 5.56. Being a new in box M400, you can shoot both and both should work fine. Be sure to break down your rifle and clean all the factory preservative from the rifle and relube, as the factory preservative can become sticky and cause failures if fired before cleaning, making the user think that the rifle is defective.
If the rifle is used or custom built, check the stamp marks on the barrel and not the lower receiver, as it could be chambered differently.
The way ammo is getting harder to find, you should still use 5.56 for the first couple of hundred rounds to get it broke in and then .223 should be fine. That is just my recomendation and you should use your own discretion.

You can shoot .223 in a rifle chambered for 5.56 but you should not shoot 5.56 out of a rifle chambered in .223. That being said, the M400 is chambered for 5.56. Being a new in box M400, you can shoot both and both should work fine. Be sure to break down your rifle and clean all the factory preservative from the rifle and relube, as the factory preservative can become sticky and cause failures if fired before cleaning, making the user think that the rifle is defective.
If the rifle is used or custom built, check the stamp marks on the barrel and not the lower receiver, as it could be chambered differently.
The way ammo is getting harder to find, you should still use 5.56 for the first couple of hundred rounds to get it broke in and then .223 should be fine. That is just my recomendation and you should use your own discretion.

Thank you!

Will the rifle be pretty easy and understand able to tear down and lube? Any specific portion I need to lube, or any I should not? The manual will be helpful but confusing and over whelming at times.

Just buy a spray can of BreakFree CLP at walmart in the sports department and remove the bolt carrier group from the rifle and spray the heck out of it and then cover up the chamber with a finger and sprays the guide area where the bolt carrier goes into and then put the wet bolt carrier group back in your rifle and you are good to go. You should shoot her wet for a while till she gets good and broke in..

As for ammo you have a choice of .223REM or 5.56X45 NATO. For just basic shooting, just get what you can get your hands on, but for sniper like accuracy, get some heavy 5.56 or .223 in the 62 grain and up weights. The barrel has a 1:7 Twist rate and therefore is designed to stabilize the flight of a heavier bullet. You should get reasonable accuracy with cheap 55grain ammo with groups in the 1 to 4 inch range at 100+ yards. Have fun with her and keep her clean. The key to the AR15 is to keep everything wet but the chamber and barrel at least until it breaks in then you can focus mainly on shiny bare metal parts indicating points of contact or bearing surfaces.

I own 2 M440's and a 516. All were srtipped down when new and given the top to bottom cleaning and re-lubed with TB25 grease. Close to 900 roundes of Misc 5.56 and 223 ammo and not one issuse with any of the guns. I do stay away from the cheap ammo though.